Human cyclic ADP-ribose synthetase (CD38) is an ectoenzyme, expressed on the surface of β cells, that makes cyclic-adenosine-diphosphate-ribose (cADPR) and ADP-ribose from nicotinamide-diphosphate-ribose (oxidized form) (NAD+). Nicotinamide guanine dinucleotide (NGD+) is also cyclized by CD38 to form cGDPR, and hydrolyzed to form GDPR. Human CD38 (1–2, 33) and Aplysia californica adenosine-diphosphate (ADP) ribosyl-cyclase (3–4, 34–35) share 68% homology in primary sequence (36); both synthesize cADPR from NAD+ (15, 37). The compound cADPR is formed by intramolecular ADP-ribosylation at the N1 position of the adenine ring (5), and is a potent agent for calcium-ion (Ca2+) release from intracellular Ca2+ stores (1, 7, 38–44).
An increasing volume of evidence indicates that CD38 and ADP ribosyl-cyclase regulate important physiological processes in invertebrates (1, 38–44), plants (8), and mammals (1, 2, 6, 9–10, 45) via the synthesis of cADPR. In mammals, CD38 and cADPR have been implicated in the regulation of cellular processes, including insulin release (9), lymphocyte activation (2, 10), bone homeostasis (45), and synaptic plasticity (6). Additionally, the presence of CD38 has been linked to cytokine-induced differentiation (1), cell adhesion (1), and signal transduction (37). The wide distribution of CD38 in the brain, white blood cells, pancreas, and other tissues, suggests that this enzyme may have a general signaling role via cADPR production in vivo.
cADPR has been implicated in the release of Ca2+ from inside cells (1, 7, 38–44). Mobilization of internal calcium is an important signaling mechanism in cells, and may be implicated in numerous pathologies. Diseases and conditions associated with the transmembrane flux of Ca2+ ions into cells, particularly vascular smooth muscle cells, cardiac muscle cells, and cells of the nervous system, may include angina (e.g., angina pectoris, chronic stable angina, and vasospastic angina), arrhythmias, atrial fibrillation, hypertension, paroxysmal supraventricular tachycardia, adrenoleukodystrophy (ALD), and multiple sclerosis (MS).
Small-molecule, mechanism-based inhibitors of specific signaling pathways are desirable for therapeutic use. Unlike most conventional drugs, which temporarily inhibit the target enzyme, many mechanism-based inhibitors permanently disable the target enzyme. Small molecules that inhibit CD38 would be expected to decrease levels of cADPR, resulting in a modification in intracellular levels of Ca2+. In particular, inhibition of cADPR-stimulated Ca2+ release would be expected to have significant effects on calcium-mediated signaling pathways in many cells and tissues, thereby providing a useful treatment option for pathologies in which Ca2+ regulation is implicated. However, prior to the present invention, there were no known effective, small-molecule, mechanism-based inhibitors of CD38 having the potential for regulation of cADPR levels.